Control apparatus



Feb. 25, 1941. R, w s 2,232,753

CONTROL APPARATUS Filed March 21, 1940 8 Sheets-Sheet 1 j Veil/07'. Raiser Z. M75077 M MM We 4W Jf/orrzeys Feb. 25, 1941. R. 1;. WILSON CONTROL APPARATUS Filed March 21, 1940 8 Sheets-Sheet 2 Feb. 25, 1941. R. L. WILSON CONTROL APPARATUS Filed March 21, 1940 8 Sheets-Sheet 3 THE c Za Feb. 25, 1941. R 1 WILSON 2,232,753

CONTROL APPARATUS Filed March 21, 1940 8 Sheets-Sheet 4 R. L. WfiLSON 32,753

CONTROL APPARATUS Filed March 21, 1940 8 Sheets-Sheet 5 4/50 r22 eys R. L, HLSON CONTROL APPARATUS Filed. March 21, 19410 8 Sheets-Sheet 6 N ws W IF M mmy Nam/W M wwmm EN LU wm Iii m mm Iii/omega CONTROL APPARATUS Filed March 21., 1.940 8 Sheets-Sheet 8 Patented Feb. 25, 1941 CONTROL APPARATUS Rosser L. Wilson, Mahwah, N. J., assignor to The American Brake Shoe and Foundry Company, Wilmington, Del., a corporation of Delaware Application March 21, 1940, Serial No. 325,150

12 Claims.

This invention relates to apparatus for controlling the relative speeds of moving parts of machinery, and particularly to such apparatus when used for maintaining a substantially syn- 5 chronous relation in the rotative speeds of two or more rotating parts such as engine or motor drive shafts and the like.

It is frequently desirable to maintain the drive shafts of two or more engines or the like in sub- 10 stantial synchronism, as where such engines or the like are operated to supply power to a single operative device. Prior control apparatus for so correlating the associated drives on machinery of this character has contemplated setting up op- 15 posing mechanical, electromotive, or magnetic forces respectively proportional in magnitude to the rotative speeds of the parts to be synchronized or otherwise correlated, and the unbalancing of such opposing forces has been effective 20 through the medium of devices such as differential governors, synchronous motors, differentially wound relays, or the like, to bring about a control operation of the apparatus.

It is a salient object of the present invention 5 to correlate the operation of two or more engines or like driving means without resorting to the use of detecting apparatus responsive to the interaction of opposing forces.

In the development of aircraft such as airplanes it has been found advantageous in many instances to employ two or more engines, each driving a propeller shaft on the plane or other aircraft. One of the difficulties which has been encountered on aircraft of this kind is that undesirable low-frequency beat vibrations may be set up when the several propeller shafts do not rotate in substantial synchronism with each other, and these beat-frequency vibrations are frequently manifested in various parts of the aircraft structure in the form of concentrated hammering impulses which have a more. or less disturbing effect upon the occupants. Moreover,

such beat vibrations are likely to appear in locations where they have a detrimental effect, 5 such as in the wings of a plane, and thus they may tend to weaken the aircraft structure.

It has been proposed heretofore to automatically maintain two or more propellers on multi-engined aircraft and the like in substantial synchronism by providing a plurality of generators, each driven by a propeller shaft, and electrically connecting the generators in opposition to each other in a series circuit including an electrically operable means such as a motor which functions in dependence upon the resultant electromotive force in the series circuit to vary the speed of an engine that is to be maintained in timed relation with a master engine. An outstanding disadvantage of such an arrangement is that the generators are comparatively heavy and thus 5 considerable extra weight is added to the aircraft structure. Moreover, it is relatively difficult to design generators of small size whose electrical characteristics will be substantially identical at all speeds within the working range.

In view of the foregoing, it is an object of my invention to accurately control the operation of the propellers in aircraft and the like and to maintain the same in substantial synchronism under all conditions which may be encountered in practice, by the use of apparatus which is of extremely light, simple and inexpensive construction.

A further object is to control engine speed, for example by automatically varying the propeller pitch, through the use of novel means responsive to the relative speeds of the propellers and having a time factor for enabling operation of such pitch-adjusting or other regulating means only upon the establishment of at least a. predetermined difference in rotative speeds.

A still further object is to dispense with generators and the like for comparing propeller or propeller shaft speeds (such terms being used synonymously hereinafter) and to employ in lieu thereof electrical contact-making devices, such as commutator switches, which are embodied in a control circuit including timing means, such as slow-to-release relays, that are responsive to said commutator switches for controlling the propeller pitch-adjusting means or other engine speed governing means in dependence upon whether or not there is at least a predetermined discrepancy in propeller speeds, and in accordance with the sense of the relative rotations of the propellers, and in this regard sense of relative rotation is to be understood to mean the direction of rotation of one propeller relative to another or master propeller, or, to state this in an equivalent manner, the rotative speed of the firstnamed propeller in comparison with the speed of the master propeller. Thus, assuming thedirection of rotation of both propellers to be clockwise as viewed from the rear of the aircraft, if the first propeller is rotating more slowly than is the master propeller, then the sense of relative rotation of the first propeller is counterclockwise, because this propeller is rotating clockwise less rapidly than is the master propeller. On the other hand, if the first propeller is rotating more rapidly than is the master propeller, the sense of relative rotation of the first propeller is clockwise, because thispropellerisrotatingclockwiseatagreater speedthanisthemasterpropeller. Itisinthis manner that the just explained term is used herematter.

A still further object is to electrically interconnect at least two relatively stationary commutators; to provide each commutator with one or more brushes or other contact-making means rotatively movable relative thereto in response to the rotative speed of a respective one of at least two members which are to be operated in substantial synchronism; to establish energizing circuits through the commutator brushes to electrically operable means such as relays; to condition a selectedoneofstillotherenergizingcircuitstoa further electrically operable means for conditioning the same in accordance with the sense of the relative rotations of said members; and to render said relays eflective to complete the selected one of the last-named energizing circuits whenever there is at least a predetermined variation in the relative speeds o! the brushes of said commutators, said further electrically operated means operating when thus energised in a selected manner to bring one of said rotating members into substantial with the other or another of such members.

A further object is to provide means of the above described character for detecting the sense of the relative rotation, if any, between two independently rotatable parts of machinery or the like, and to provide further means for ascertaining when the magnitude of such relative rotation exceeds predetermined limits, said means and said further means being effective to control the rotative speed of one of said rotatable machine parts and to substantially synchronize it with the other of such rotating parts when a predetermined difference in rotative speeds arises.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments and the principle thereof and what I now consider to be the best mode in which I have contemplated applying that principle. Other embodiments of the invention embodying the same or equivalent principle may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the purview of the appended claims.

In the drawings.

Fig. l is a schematic plan view of the nose portion of an airplane showing sin-embodiment of my invention as used in maintaining a timed relation in the operation of two aircraft engines;

Fig. 2 is a diagrammatic view of a motor which is responsive to my novel control apparatus for regulating engine speed:

Fig. 3 is a diagrammatic view of an indicating device which may be associated with the control pparatus:

Fig. 4 is a diagrammatic view of a series of commutator switches and circuit interconnections therefor employed in one embodiment of my invention;

Fig. 5 is a diagrammatic view of a control relay arrangement which may be utilized in conjunction with the commutator switches shown in Fig. 4;

Figs. 6. '1 and 8 are diagrammatic views of a azasnsa modified form of the invention shown in. m 4 and 5;

Figs.9,l0and ll areviewsrespectivelysimilar tol'igs. 6,7and8andshowinganothermodified form of my invention;

Figs. 12, 13 and 14 are further views respectivelysimilartol'igs.6,7and8andshowingstili another modified form of my invention;

Figs. 15, 16 and 17 are views oi a modification similar to that shown shown in Figs. 9 to 11;

Fig. 18 is a wiring diagram of an anbodimmt in which the brushes for determining direction of relative rotation are driven independently of the brushes for effecting control when the magnitude of such relative rotation exceeds predetermined limits;

Fig. 19 is a diagrammatic view of an arrangement that might be used under certain conditions for reducing the number of conductors required in the cables that are used in conjunction with my apparatus; and

Figs. 20 and 21 are diagrammatic views of two modifications each similar to that shown in m 9 to 11.

In the form of my invention which is adapted for maintaining a timed relation in the speeds of two or more engines on aircraft and the like, one of the engines, such as El, driving a propeller asPLFig. l, isselectedtobethemaster engine, and the other engine or e nes, such as E2, each driving a propeller as P1, are maintained in substantial synchronism with this master engine by the control apparatus. Briefly, such control apparatus comprises at least one master commutator switch as SI having rotative parts driven by the master engine El; another or other canmutator switch or switches as 82, each having rotative elements which are driven by a corresponding engine as IE2 that is to be operated in timed relation with the master engine El; a control unit as CU that is governed jointly by a master switch as Si and by a secondary switch as 82 for each engine as E2 that is tobe so timed with respect to the engine El; and a motor or like means M for each engine as E2 and which is controlled by a control unit as CU for regulating the speed of the engine as E! to maintain the same in substantial synchronism with the master en gine El. Such regulation of the engine speed by the motor M can be accomplished in a number of ways, for example by affecting the rate at which fuel is supplied to the engine E2, but at present I prefer the well-known arrangement in which variable pitch propellers are employed on an airplane or other aircraft and. wherein the speed of each engine may be affected in. a dedred amount by adjusting the pitch of the propeller driven thereby.

Referring to the wiring diagram of the motor M in Fig. 2, said motor is preferably of the directcurrent type operable on low voltage and is provided with two alternatively energizable field coils 2| and II for driving the armature 22 of the inc tor M in either a forward or a reverse direction. When circuit is completed from a source of voltage through a conductor 21 to the field coil 2., in a manner to be explained, and thence through the winding 24 of a rday 25, conductor 26, rheostat 21, and conductor 28 back to the source of voltage, field coil 2| is energized for driving the motor M, say, forwardly, and at the same time the relay 2! closes its normally open contact IIA to complete circuit from the same or another voltage source through conductor 2!, a normally II /the pitch of the propeller P2.

closed cam contact 30, conductor 3|, relay contact 25A, conductor 32, winding of armature 22, conductor 33, a normally closed cam contact 34, and conductor 35 back to the voltage source. Normally the motor M is then driven forwardly until such time as circuit to the field coil 20 is broken, whereupon relay 25 is deenergized and opens its contact 25A.to break the circuit to the armature 22 and thereby stop the operation of the motor M, thi arrangement insuring that the motor will have no tendency to race when the field is demagnetized. As the motor turns, it

actuates a means' which, in the present instance, causes a variation in the pitch of the propeller P2, Fig. 1, and such means may comprise, for example, a worm rack operable through suitable devices to change the pitch of the propeller and effect a corresponding adjustment of the engine speed, or the motor M can expeditiously serve to actuate a fluid pressure system such as is shown, for example, in United States Letters Patent No. 2,188,313, issued January 30, 1940, for varying Preferably the mechanical reduction ratio afforded by the pitchadjusting means is great enough so that, in con- .nection with certain other features of the control apparatus described hereinafter, it serves to insure that there will be no tendency for the device to hunt before attaining a condition of substantial synchronism.

In a manner fully described hereinafter, the field coil 20 of the motor M can normally be energized to cause the pitch of the propeller P2 to be varied in a predetermined direction only when the rotation of the propeller P2 relative to the propeller PI is in a certain sense, say counterclockwise, assuming the propellers PI and P2 to be rotating in a clockwise direction, as viewed from the rear looking toward the nose of the aircraft shown in Fig. 1, such energization of the field coil 20 being effected under the control of the control unit CU, Fig. 1. However, when the rotation of the propeller P2 relative to the propeller PI is in a clockwise sense, still assuming both of the propellers PI and P2 to be rotating clockwise as viewed from therear of the aircraft, the control unit CU may establish a circuit from the aforesaid source of voltage to a conductor 36, the field coil 2|, winding 24 of relay 25, conductor 26, rheostat 2'|,'and conductor 28 back to the voltage source, and this is effective to drive the motor M in its reverse direction in a manner similar to that described hereinabove for forward rotation of the motor M. Thus the motor M can actuate the pitchadjusting means of the propeller P2 either in a forward or in a reverse direction for bringing the engine E2 into substantial synchronism with the engine El.

11, for example, it is required that the engine E2 be slowed down relative to the engine El, the pitch of the propeller P2 should be increased to thereby increase the load on the engine E2, and this is accomplished in ,the present instance by energizing the field coil 2| to drive the motor M in its reverse direction. On the other hand, if the speed of engine E2 is to be increased, the load on this engine is decreased by energizing the field coil 20 of the motor M to thereby drive the motor forwardly and thus decrease the pitch of the propeller P2 and consequently the load on the engine E2.

The cam contacts 30 and 34 serve as limit switches for breaking circuit to the armature 22 in the event the propeller P2 is adjusted to one or the other of its extreme angular positions. It will be understood that while I have described the motor M as being a direct-current motor equipped wlth'separate field coils for forward and reverse rotation, it would be equally feasible to employ a single field coil with a reversing,

switch arrangement that would be responsive to the alternative potentlalizatlon of the conductors 23 or 36. It is likewise manifest that an alterhating-current motor could be employed in lieu of a direct current motor, the reversible shading-coil type of motor being satisfactory for this purpose.

It may be deslrable to furnish some indication to the pilot of the aircraft when the rotative speeds of the propellers as PI and P2 differ in a substantial amount and to also indicate in what sense the propeller as P2 is rotating relative to the master propeller as Pl. To this end, lamps 31 and 31, Fig. 3, may be provided on the instrument panel for each propeller as P2. Preferably these lamps are operated under the control unit CU, Fig. 1, and this may be accomplished by connecting one terminal of the filament 38 of the lamp 31 to the conductor 23, Fig. 2, by a conductor 39, the other terminal of the filament 38 being connected by a conductor 40 to one terminal 4| of the filament 42 of lamp 31, and thence through a conductor 43, Figs. 3 and 2, to the conductor 28. The other terminal of the filament 42 is connected by a conductor 44 to the conductor 36, and thus whenever circuit is closed by the control unit CU to the field coil 20, the lamp 31 will be lighted, but if circuit is closed to the field coil 2|, the lamp 31 will be lighted. While I have illustrated a visual signal means for the purpose of indicating relative rotations of the propellers, it is equally expedient to employ an audible signal, or both.

Referring now to the embodiment of the invention shown in Figs. 4 and 5, each of the switches as SI and S2, Fig. 1, comprises a commutator generally designated 45 stationarily mounted on the frame of the associated engine as El, this commutator being composed of a number of segments 46 which are insulated from each other and are arranged around the circumference of a circle. The switch SI of the master engine El is provided with a single brush 4? that is rotatably mounted so as to make wiping contact with the segments 48 of the commutator 45 of the switch Sl. Brush 41 is adapted to be driven by the propeller shaft of the master engine El either by being mounted directly there on or by being geared thereto. The switch S2 includes a commutator 45 stationarily mounted on the engine E2 and rotatable cooperating brushes 48, 49 and B which are maintained in fixed circumferentially spaced relation to each other and are driven by the crank shaft of the engine E2. ihe commutators 45 and 45' of the switches SI and S2 respectively are interconnected segment for segment by conductors included in a cable generally designated 5i, Figs. 1 and 4, which extends between the switches Si and S2.

If the aircraft includes further engines in addition to those shown in Fig. 1, each such engine will have associated therewith a commutator switch as S3, S4, and so on to Sr, and all of .the commutators of these switches will be interconnected segment for segment with the commutator 45 of the switch SI in the same manner as is the commutator 45' of the switch S2, which is to say by conductors in the cable 5|.

Thus a flexible electrical connection is provided between the master switch SI and the switches as S2 of the engines as E2 that are to be substantially synchronized with the master engine El.

The brush 4'! of the switch SI is maintained in electrical contact (for instance, bymeans of a slip ring or the like) with a conductor 52, Figs. 4 and 5, which leads to a terminal 53 that is connected by a conductor 54 to one terminal 55 of a source of voltage B. The other terminal I! of the voltage source B is respectively connected by conductors 51 and 58 to one side or each of the windings 59 and Oil of a pair of slow-to-release relays SRI and SR2. The other side of the winding 59 of the relay SRI is connected by a conductor 8| to the brush ll of the switch 82, Fig. 4. Similarly, the other side of the winding ill of the relay SR2 is connected by a conductor 52 to the brush 50 of the switch S2. Brush 5 is electrically connected through a conductor 52 to the terminal of a contact 84 of the relay SR2, which contact is normally in engagement with another contact 85 when relay SRI'is deenergized. Contact 65 is connected by a conductor 20 to one side of the winding 51 of a quick-acting relay R, and the other side of this winding is connected through a conductor 55 to the terminal 65 and thence by a conductor 15 to the terminal 52 of the voltage source B.

If the propellers PI and P2, Fig. 1, are rotating in exact timed relation, there is no relative rotation of the brushes of the switches Si and S2. However, when the propeller P2 rotates relative to the propeller Pl, there is relative rotation of the brushes #8, l9 and 50 of the switch S2 with respect to the brush .41 of the switch SI and in this event the brush 41 is successively brought into electrical contact with the brushes ll, 49 and 52, the sequence in which such contacts are made being determined by the sense of the rela tive rotation of the propellers Pi and P2, as will now be explained. Assuming that normally the direction of rotation of the propellers is such that the brushes of the switches as SI and S2 rotate clockwise as viewed in Fig. 4, if the propeller P2 rotates more slowly than does the master propeller PI, the brushes of the switch 82 will rotate counterclockwise relative to the brush 4'! of the switch SI. Under these circumstances,

' the brush 41 may first be brought into electrical contact with the brush 8 whereupon circuit is momentarily completed from the terminal 55 of the voltage source B through conductor 54, terminal 52, conductor 52, brush 41, conductors of cable 5|, brush 8, conductor 6|, winding 59 of the slow-to-release relay SRI, and conductor 51 to the terminal 56 of the source B of electric current. Relay SRI thereupon energizes and closes its contact ll. Continued counterclockwise relative rotation of the brushes of switch 82 causes the brush H to be next brought into electrical contact with brush 8 and circuit is thereupon extended from the terminal 55 of the source B of electric current in a manner similar to that just described through the brush 4! and conductor 82 to the normally closed contacts H and 65 of the relay SR2 to the conductor 55 andthence through the winding 81 of relay R, conductor 58, terminal 58 and conductor Hi to the terminal 56 of the source B of electric current. Relay R thereupon energizes and remains energized so long as brush 41 remains in electrical contact with the brush 49. Relay SRO, being slow to release, maintains its contact ll closed for a predetermined length or time after the brush 4! moves out of electrical contact with the brush ll; hence it the rate of relative rotation of the propeller P2 is great enough, the brush 1 of switch SI may be brought into electrical contact with the brush 5!! of switch S2 prior to the time when relay SRI releases its contact 1|. Brushes l8 and 50 are spaced comparatively close together about the periphery of the commutator 45' and the brush 41 is of such width that it is brought into electrical contact with the brush 50 prior to moving out oi! electrical contact with the brush 48. Therefore, relay R remains energized while circuit is completed from brush 41 through the brush 50 and conductor 82 to the winding 60 of relay SR2 and thence through conductor 58 to the terminal 52 of the source of electric current. Relay SR2 in energizing closes circuit between its contact 55 and another contact 12 prior to separating the contact 64 from the contact 85, the contacts 12, 55 and 54 thus constituting make-before-break contacts. Such closure of the contact 65 with the contact 12 completes a circuit from terminal 55 of the voltage source B through a conductor 13 and a contact ll of the relay R, which contact is closed while relay R is energized, and conductor 15 through the contacts 12 and 65 to the conductor 66 to thereby provide a holding circuit for the relay R so long as relay SR2 remains energized. When relay R was energized upon establishment of a circuit through brushes 4! and 4!, it separated a movable contact ll thereof from a stationary ilxed contact 11, with which it is in engagement when relay R is deenergized, and brought this contact 16 into engagement with another stationary contact 18 which is connected to the conductor 23, Figs. 5 and 2. Energization of the relay SR2 in the manner described closes contact 19 of this relay and assuming that the degree of relative rotation of the propeller P2, Fig. 1, is suillciently great that the relay SR2 is energized prior to the time relay SRI has released its contact 1i, circuit is completed from the terminal 55 of the voltage source B, through conductor 54, terminal 53, conductor 80, relay contact II, conductor ll, contact 19 of relay SR2, conductor 82 and contacts 16 and 18 of the relay R to the conductor 23 and thence circuit is extended in the manner hereinabove described through the field coil 20 of the motor M to the conductor 28 which leads to the terminal 59, Fig. 5, that is connected by the conductor 10 to the terminal 55 of the voltage source B.

It will be seen from the foregoing that whenever the propeller P2 is rotating relative to the propeller PI in such a manner that the brushes of the switch S2 rotate counterclockwise relative to the brush 41 of the master switch S i, as viewed in Fig. 4, the relays SRI and SR2 are alternately energized and if the degree of relative rotation is such that relay SR2 is energized within the release time of the relay SRI, an energizing circuit is completed through the contacts H and 18 of the relays SRI and SR2 and the contacts 16 and ll of the relay R to the field coil 20 of the motor M for thereby driving the motor M in a forward direction, which in the present instance suffices to increase the speed of the engine E2 (either by reducing the pitch of the propeller P2 to lighten the load on the engine E2, or by regulating the speed of the engine E2 in any other equivalent manner) for thereby decreasing the counterclockwise relative rotation of the promeans peller P2 with respect to the propeller Pl, as viewed from. the rear of the aircraft. Thus, the control unit CU, Fig. 5, is responsive to at least a predetermined variation in the rotative speed of the propeller P2 relative to the propeller Pl, as detected by means of the commutator switches as SI and S2, Fig. 4, for correcting the relative rotative speed of the propeller P2.

The brushes 48, 49 and 50, Fig. 4, continue to rotate counterclockwise relative to the brush 41 so long as there is any such relative rotation of the propeller P2, and as brush 41 moves out of electrical contact with brush 50, the energizing circuit to the winding of the relay SR2 is broken to thereby initiate deenergization of this relay. If brush 41 then electrically contacts brush 48 prior to the time relay SR2 has released its contact 19, the relay SRI is energized to eifect or maintain closure, as the case may he, of the contact 1| thereof, and the above described energizing circuit to the winding 26 of the motor M continues.

For a reason which will appear hereinafter, the release times of the relays SRI and SR2 and the brush and segment widths are so proportioned that it will not be possible, upon initiation of such relative rotations as may be expected to develop in practice, for a brush as 50 to move out of electrical contact with the master brush 41 and back into electrical contact with the master brush 41 (at least during the first relative turn of the brushes as 50 with respect to the brush 41) within the period during which the relay as SR2 remains in an operative condition after it has been initially energized by the first electrical contact between brushes as 41 and 58. Thus, the average relative velocity of the brushes as 50 with respect to the master brush 41 must have at least a predetermined value in order that the relays SRI and SR2 may be maintained in a continual state of concurrent operation, and depending upon whether or not the relative acceleration persists long enough to produce this average relative velocity, an energizing circuit to the motor M will be either continuously or intermittently established by the relays SRI and SR2. In no event, however, will a relay as SR2 be maintained in an operative condition throughout the entire initial relative turn of a brush as 50 with respect to the master brush 41 in a given direction.

After the first rotation of the brushes as 50 relative to the master brush 41 has been completed, if relative acceleration of the propellers PI and P2 continues, this may cause the relative rotation to become great enough for the relays SRI, SR2 and R (still assuming counterclockwise relative rotation of the brushes 48, 49 and 50) to remain in a continual state of energization, relay R being held locked up through its aforesaid holding circuit controlled by relay SR2, and the field coil as 20 and the armature 22 of the motor M are therefore maintained energized for continuously operating the motor M until the speed of the engine E2 has been varied sufliciently to bring the average relative velocity of the propellers PI and P2 below the aforesaid value as determined by the time factor embodied in the slow-to-release relays SRI and SR2. Thereafter, the control unit CU continues to actuate the motor M for varying the velocity of the propeller P2 relative to the master propeller Pi, but in this instance circuit to a motor field coil as 28 and armature 22 cannot be maintained throughout the entire time interval between two successive electrical contacts of a brush as 41 with a brush as 56, inasmuch as one or the other of the relays SRI or SR2 will be released in this interval of time. Hence, as the brushes rotate relative to each other, an energizing circuit is repeatedly established by the relays SRI and SR2 to a motor field coil as 26. The i'requency with which the motor M is thus repeatedly actuated obviously decreases while a condition of substantial synchronization is being approached, since the brush 41 does not repeatedly contact the brushes 48 and 56 at as great a rate at low relative speeds. Thus, the motor M is comparatively infrequently actuated at low relative speeds approaching zero, and this, in combination with the high reduction ratio and negligible momentum of the motor drive, renders the likelihood of the apparatus hunting while the propellers are being brought into timed relation extremely remote.

It will be appreciated that there might be an instance in which the relays SRI, SR2 and R are energized for correcting relative rotation of the propeller P2 such as causes counterclockwise relative rotation of the brushes as 50, Fig. 4, as just described, and wherein substantial synchronization is restored within the time interval while the relays SRI and SR2 are still concurrently operative. However, this would only be likely to occur where the relative velocity was very small to begin with, and under such circumstances the overlap in release times of the relays SRI and SR2 is relatively slight, so that in addition to the factthat the motor M is infrequently energized when the propeller P2 is approaching substantial synchronism with the propeller Pl, the duration of each such energization period of the motor M becomes correspondingly small. in: any event the propellerl P2 would not be brought far enough past a condition of substantial synchronism. to cause hunting of the control apparatus.

In the event the propeller P2 rotates more rapidly than the propeller Pl, still assuming that both propellers are rotating in such a direction as to drive the brushes of the switches SI and S2 clockwise as viewed in Fig. 4, the brushes 48, 49 and 50 of the switch S2 associated with the propeller P2 then rotate clockwise relative to the brush 41 of the master switch SI. In this case the brush 41 may be brought into electrical contact first with, say, the brush 48, thereby energizing the slow-to-release relay SRI. If the clockwise relative rotation of the propeller P2 continues, the brush 41 then breaks contact with the brush 48 and is brought into electrical contact with the brush 50 to thereby energize the slow-to-release relay SR2. Continued relative rotation of the brushes causes the brush 4! to be brought into electrical contact with the brush 41 while the latter is still in electrical contact with the brush 50. When relay SR2 energizes, it separates its contacts 64 and 85 so that when circuit is extended from the voltage source B through brush 41 and the brush 49 to thereby apply potential on the conductor 63, there is no further effect insofar as the relay R is concerned because the connection from the conductor 63 to the conductor 66 which leads to the winding of the relay R is broken when the aforesaid contacts 64 and 65 are separated. Hence under these circumstances the relay R does not energize and the contacts 16 and 11 of this relay remain in engagement while the contact 18 of this relay remains separated from the contact 10. Assuming that the degree of relative rotation is such that the slow-torelease relay SR2 is energized within the release time of the relay SRI, a circuit is completed as aforesaid from the terminal 5! of the voltage source 8 through conductor 84, terminal 54, conductor 44, contact 1| of relay SRI, conductor 8|, contact 10 of relay SR2, conductor 82 and contacts 16 and 11 of the relay R to the conductor 24, Figs. 5 and 2, and thence through the field coil 2| of the motor M, winding 24 of relay 2!, conductor 26, rheostat 21, conductor a, terminal 48, and conductor 10 to the terminal 54 of the source of voltage B. This results in the energization of the field coil 2| and of the relay 2| to thereby initiate reverse rotation of the motor M and this is effective to adjust the speed of the engine E2, for example, by varying the pitch of propeller P2. in such a manner as to tend to correct the rotative speed of the propeller P2 relative to the propeller Pl.

As explained hereinabove in connection with the operation for correcting low relative velocity of the propeller P2, the just described operation for correcting high relative velocity of this propeller entails continuous operation of the motor M at a comparatively high relative velocity of the propellers, but an intermittent actuation of the motor M while a state of substantial synchronism is being approached, such latter operation of the motor M being repeated often enough to bring the propeller P2 and/the engine E2 into substantial synchronism with the master propeller PI and the master engine El, it being recalled that the release times of relays SRI and SR2, Fig. 5, are such that the energizing circuit to a motor field coil as 2| cannot be maintained throughout the first complete turn of the brush 41 of the switch Si relative to the brushes of the switch S2. The only substantial diiference between the just described operation of the control unit CU for correcting high relative velocity of the propeller P2, as compared with the operation for correcting low relative velocity of this propeller, is that the relay R remains reenergized at all times in the former operation inasmuch as the master brush 41 will always electrically contact the brush it to energize the relay SR2 at a time when the relay R is in a deenergized state and, unless substantial synchronization is eiIected after the brush 41 breaks contact with the brush ll and while it remains in electrical contact with the brush 44, the relay SR2 will remain energized to keep its contacts 44 and 45 open at least until the brush 4! moves out of electrical contact with the brush 41 so that an energizing circuit cannot be completed to the winding 81 of relay R.

It will be appreciated from the foregoing description that when the brushes of the secondary switch B2 are rotating clockwise relative to the brush 41 of the master switch SI, as viewed in Fig. 4, the relay R normally should remain deenergized to thereby condition a circuit to the reverse field coil 2| of the motor M. On the other hand, if the brushes of the switch S2 are rotating counterclockwise relative to the brush 41 of the switch SI, the relay R should be energized to enable the forward field coil of the motor M to be energized. It's to be noted in this connection that a condition of substantial synchronization might exist while the brush 41 is in electrical contact with the brush ll but is out of electrical contact with the brush 4! so that the relay SR2 alone is energized. Then, if the propeller P2 were to commence relative rotation in such a direction that the brushes 44, 4! and $4 of the switch S2 start to rotate counterclockwise relative to the brush 41 of the switch SI. as viewed in Pig. 4, the brush ll would move out of electrical contact with the master brush 41 and the brush 4. would be brought into electrical contact with the brush 41 to thereby energize relay SRI, and if this occurred within the release time of the relay SR2, a delay in the operative response of the control unitGU would be effected inasmuch as the relay R is not energized at the time the relays SRI and SR2 are maintaining their contacts 1| and 1! closed concurrently, so that an energizing circuit would be momentarily established from the voltage source B through the aforesaid contacts 1| and 1! to the contacts 14 and 11 of the relay R and thence through the conductor 34 to the reverse field coil 2| of the motor M. It has also been noted hereinabove, however, that the release time of the relay SR2 is such that even under the most extreme conditions that might be foreseen in practice, the relay SR2 will not remain energized throughout the interval between two successive electrical contacts of a brush 5. or 44 with the brush 41 during the first relative turn of these brushes. Moreover, the apparatus should so operate that under such extreme conditions the relay SR2 would open its contact 19 and close its contacts 64 and 5 prior to the time the brush 49 moves into electrical contact with the brush 41.

Thus, the release times of the relays and the brush widths may be so selected that under any circumstances the relay SR2 is fully deenergized at the time brush 41 electrically contacts the brush 49 during the first counterclockwise rotation of the brush II of switch S2 relative to the brush 41 of the switch SI, Fig. 4. Hence, despite any operative delay which might be experienced under the conditions Just described, relay R will thereafter become energized and if the condition of counterclockwise relative rotation of the brushes of switch S2 persists, the relay R will be energized each time the relay SR2 assumes a state of energization, as described hereinabove, and circuit is thus conditioned to the forward field coil 24 of the motor M.

It will be understood that the control unit CU, Figs. 5 and 1, is associated with the switches S| and S2 for controlling the speed of the engine E2 relative to the master engine El. If there are additional engines to be synchronized with the master engine El on the aircraft, each such engine is provided with a switch such as S2 or S4 and a control unit identical with the unit CU is associated with such switch and the master switch SI for controlling the operation of a motor similar to the motor M associated with the particular engine to be synchronized.

In the circuit shown in Fig. 4, there is a possibility, particularly where there are a large number of engines to be synchronized, that a bridging condition may occur when the brushes on the various commutators assume certain relative positions. To avoid this feature, an arrangement shown in Figs. 6 and '7 may be resorted to and by referring to these figures, it will be seen that the master engine El, Fig. 1, has associated therewith a series of master switches Sla, Sla', Sin", and so on. Each of these master switches is identical in construction with the switch Sl shown in Fig. 4 and the brushes 41a of the various master switches are connected in parallel by a conductor 52a to a terminal Ila and thence by a conductor 84a to the terminal 55a of the voltage source Ba, Fig. 8, the parts bearing the same reference characters and differing only in the suffix attached thereto being identical with corresponding parts in Fig. 5. Conductors contained in cables as 5Ia, 5Ia' and 81a" intercon-' nect the commutators as 4511 of the master switches as SIa, SIc', and Ski" respectively with the commutators as 'a. of the switches S241, S311 and 54a associated with the engines as E2, Fig. 1, that are to be maintained in timed relation with the master engine EI, the commutator of each switch as S2a being thus interconnected segment for segment with its corresponding master switch commutator as 45a. Each switch as 82:; is similar in construction to the corresponding switch as S2 shown in Fig. 4 except that in this instance the brushes as 58a. are somewhat wider than are the brushes 58.

With the foregoing arrangement there is no likelihood that undesirable bridging conditions will occur, but in all other respects the apparatus schematically shown in Figs. 6, 7 and 8 operates in the same manner as that shown in Figs. 4 and 5, those elements which perform the same functions in the two circuits being numbered alike except for the suiilxes on the reference characters. I 1

In the circuit shown in Figs. 9, 10 and 11 the arrangement of the master switches Slb, Fig. 9, associated with the engine EI, Fig. 1, with relation to othercommutator switches as S2b, Fig. 10, is generally similar to that shown in Figs. 6 and 7, which is to say that each switch as S2!) is inter connected with its corresponding master switch as SIb by conductors in a. cable as 5Ib. The switches as S2e, however, differ in construction from the switches S2 in that each switch is provided with three brushes 85, 88 and 81, which may all be contained within a semi-circle as shown in Fig. 10. Each switch as 82b has associated therewith a control unit as CUb which is somewhat similar in construction to the control imit CU shown in Fig. 5, like elements being numbered with the same reference character in Fig. 11, but bearing the suffix b. Thus the brush 88, Fig. 10, is connected by a conductor 6Ib to one side of the winding 59b of the relay SRIb, the other side of this winding beingconnected by a conductor 51b to a terminal 58b of the voltage source Bb. The conductor82b, which is electrically connected to the brush 85 in this instance, is connected to one terminal of a normally closed contact 88 of a quick-to-release relay R2. If during relative rotation of the brushes 85, 88 and 81 with respect to the brush 41!), the brush 41b electrically contacts the brush 85 at a time when the relay R2 is deenergized, circuit is extended from the terminal 55b of the voltage source Bb through the brushes 41b and 85 to the conductor 82b and the closed contact 88 and thence through a conductor 89 to a terminal 98 from whence a conductor 9| leads to one side of the winding 92 of another quick-to-release relay RI, the other side of this winding being connected by a conductor 93 to the terminal 58!) of the source Rb, and under these conditions the relay RI is energized to open its contact 9 1 and close its contact 95. Such closure of the contact 95 extends circuit from the terminal 98 which is potentialized, so long as brushes 41b and 85 are in electrical contact, through conductor 98, contact 95, conductor 91 to a terminal 98 and thence through a conductor 99 to a contact I88 of the slow-to-release relay SRIb.

Brush 41b is of such width that it can never be in electrical contact with more than one of the brushes 85, 88 or 81 at any instant. Hence, assuming that there is counterclockwise rotation of the brushes 85, 88 and 81 relative to the brush 41b, the just described energization of the relay RI is only momentary, if relay SRIb has not been energized to maintain its contact I88 closed at the time the relay RI is energized, for when brush 81b moves out of electrical contact withthe brush 85, circuit from the source Bb to the conductor 82b is broken to remove this source of energization from the winding 92 of the relay RI and the only other source of energization for this winding is through the contact I88 of the relay SRIb which, as has just been explained, is open until the brush 41b electrically contacts the brush 88 during relative counterclockwise rotation of the brushes 85, 88 and 81, as viewed in Figs. 9 and 10 so thatrelay RI does not remain energized.

When brush 41b is brought into electrical contact with the brush 88, a circuit is extended from the voltage source Bb through brushes 41b and 88 to the conductor 6Ib which leads to the winding 59b of the slow-to-release relay SRIb, the other side of this winding being connected by a conductor 51b to the source Bb. Relay SRIb thereupon energizes and closes its normally open contacts I88 and 1Ib. As brush 41b moves out of electrical contact with the brush 86 during continued relative rotation of the brushes, the relay SRIb commences to deenergize but being slow to release it maintains its contacts H2; and I88 closed for a predetermined time after deenergization of this relay is initiated. Assuming that the degree of relative rotation of the propellers is such that brush 81 moves into electrical contact with the brush 411) during the aforesaid interval, circuit is extended from the source Bb through the brushes 47b and 81 to the conductor 83b which leads to the normally closed contact 9| of the relay RI, which relay, as has just been explained, remains deenergized when the brushes 85, 88 and 81 are rotating counterclocking relative to the brush 41b and during the time the brush 41b is not in electrical contact with the brush 85. Hence, circuit is further extended from the contact 94 through a conductor I (H to a terminal I82 and thence through a conductor I83 to one end of the winding I84 of the relay R2, the other end of this winding being connected by a conductor I85 to a terminal 89b which is connected by conductor 18b to the voltage source Bb. Relay R2 thereupon energizes to open its contact 88 and close its contacts I88 and I81. Closure of the contact I88 establishes a circuit from the terminal I82 through a.conductor I 88, contact I88 and conductor I09,to the terminal 98. It having been assumed that the relay SRIb is maintaining its contact I88 closed at the time relay R2 is thus energized, circuit continues from terminal 98 through conductor99 and contact I88 to aconductor II8 leading to a terminal I I I which is included in the conductor 881) so that circuit from the terminal 56b of the source ED is now completed as described through the winding I84 6 of relay R2, contact I88 of this relay and contact I88 of relay SRIb back to the terminal 55b of the source Bb, and a holding circuit is thereby established for maintaining the relay R2 energized so long as relay SRIb does not release its contact I88. Inasmuch as the contact 1Ib of relay SRIb is closed while this relay remains in an operative condition, circuit is extended from the terminal 55b of the source Bb through conductor 54b, terminal 53b, conductor 88b, terminal III, conductor 2, contact lib, conductor lib, terminal III, conductor ill and contact I" of relay R! to the conductor 23b which leads to a forward field coil as II of a motor as M, Fig. 2, circuit being thereafter completed through the conductor Ilb back to the terminal lib of the source Bb. Thus whenever the relay R2 is energized prior to the timethe relay SRib has been released. it is held locked up until relay SR") is released and in so doing it completes an energizing circuit to the ileld coil as 2| of a motor as M, which motor thereupon functions to correct counterclockwise (as viewed from the rear of the aircraft) relative rotation of a propeller as P2, Fig. i.

If brush 8! should electrically contact the brush 41b under these circumstances while the relay SRib is still maintaining its contacts Ill and lib closed, circuit may be extended from the source Bb through the brushes lb and I! and conductor 62b to one terminal of the contact It of the relay R2, but inasmuch as this relay is being held locked up as aforesaid, circuit cannot be completed to the winding 02 of the relay Ri and therefore relay RI remains deenergized. As in the case of the circuit shown in Fig. 5, the slow-to-release relay SR") is so timed that, under contemplated working conditions, it will not remain energized throughout the entire interval between the first two successive electrical contacts of the brush It with the brush 41b during initiation of relative rotation in a given direction, although it may thereafter remain constantly energized if the relative velocity increases, and hence if there is counterclockwise relative rotation of the brushes of the switch 82b, Fig. 10, the relays SR") and R2 may be maintained continually energized at high relative velocities, but at low relative velocities they are repeatedly energized in a manner such as described to repeatedly energize the ileld coil as 2. of the motor as M, Fig. 2, and as in the case of the control unit CU, Fig. 5, such repeated energizations of a fleld coil as 2| become increasingly infrequent and smaller in magnitude as a state of substantial synchronization is approached inasmuch as the amount of overlap of the release time of the relay SRib subsequent to the instant when relay R2 is energized, is gradually reduced when the relative speed of rotation of the brushes is being diminished.

If there is clockwise rotation of the brushes I5, 85 and 81 relative to the brush "b, as viewed in Figs. 9 and 10, the brush '1 may first move into electrical contact with the brush 41b to thereby cause energimtion of the relay R! but inasmuch as relay R2 is quick to release it immediately deenergizes when electrical contact between brushes 41b and l! is broken and prior to the instance when brush It electrically contacts brush 41b to thereby energize relay SRlb. Continued relative rotation of the brushes in this manner brings the brush '5 into electrical contact with brush 41b and if this occurs within he release time of the relay SRib relay RI is energized and the holding circuit is established for this relay through its contact It and the contact Hill of the relay SRlb in the same manner as the holding circuit was established for the relay R1 during counterclockwise relative rotation of the brushes of the switch 82b, Fig. 10, with respect to the brush "D, Fig. 9. Thus circuit is extended from source of voltage Bb through the contact lib of relay SRlb, conductor tlb, terminal ill, conductor Iii and a contact II, which is closed when the relay BI is energized to conductor lib which leads to a reverse field coil as II of a motor as M, Fig. 2, and the motor M is thereupon actuated to correct the rotative speed of the propeller P2 relative to the propeller Pi.

In the arrangement shown in Figs. 12, 13 and 14, a plurality of master switches as Sic, Hg. 12, are employed, which are similar to the master switches 'SIa shown in Fig. 6, these master switches being connected by individual cables as ilc to their corresponding commutator switches as Sic which are associated with the engines to be synchronized. The switches as Sic shown in Fig. 13, however, diiler from any hereinabove described in that only two brushes as I20 and III are included in each of these switches. The brushes as i2. and III are spaced apart at an angle considerably less than 180 degrees so that there is a much greater travel irom one brush to the other brush in one direction of relative rotation than there is in the opposite direction. The control unit CUc, Fig. 14, is identical with the unit CU in Fig. 5 in all respects except that in place of the quick-acting relay R a slow-torelease relay SR3 having the same arrangement of contacts is employed and the conductor "c is connected to conductor lilc which is under control of the brush ill. Assuming that the brushes I20 and III are rotating counterclockwise reiative to the master brush "c, as viewed in Figs. 12 and 13, the brush l2! may first be brought into electrical contact with the brush 41c and thereupon circuit is completed irom terminal 550 of the source Be through conductor 54c, terminal 530, conductor 52c, brush "0, conductors in the cable Sic, brush III and conductor I22 to a terminal I23 to which is connected a conductor Glc leading to one end of the winding 580 of the relay SRic, the other end of this winding being connected by a conductor llc to the terminal 56c of the source Be and relay SRlc is thereupon energized to close its contact 'llc. Circuit is also extended from the terminal it! through conductor 63c to the contact c of the relay SR2c, which is normally engaged with its cooperating contact 650 and circuit then continues through conductor tie to the winding ill of the relay SR3 and thence through conductor 68c, terminal 69c and conductor llic back to the terminal 560 of the source Bc so that relay SR3 is energized simultaneously with the relay SRlc. This causes the movable contact lie of the relay SR3 to move out of engagement with the stationary contact 11c and into engagement with the other stationary contact lie for conditioning circuit to a forward field coil such as II of a motor such as M, Fig. 2.

As the brush iii continues to rotate counterclockwise relative to the brush "0, Figs. 12 and 13, brush i2! eventually contacts brush 41c and circuit from the source Be is extended through these brushes to the'conductor 62c and thence through the winding c of the relay SRZc back to the source Bc. Relay SR2c thereupon energizes to separate the contacts c and 5c and engage the contact 65c with the contact 120 and if this occurs before the relay SR3 has been released, a holding circuit is established for the winding I24 oi the relay SR3 through the contact lie of this relay and the contacts 65c and 120 of the relay SRIc. Furthermore, if relay SR2c is energized within the release time of the relay SRic, a circuit is completed from the source Bc through the closed contact lie of the relay SRIc, closed contact lie or the relay SR2c and contacts 160 and 10c of relay SR3 to the field coil as 20 of the motor M, Fig. 2, to thereby actuate said motor for correcting the speed of the engine with which it is associated.

Relays SRIc, SRZc and SR3c are so timed that they will be released, even under extreme operating conditions, in the interval which elapses while a brush as "c is traversing the larger relative are between the brushes I20 and I2I during the initial relative turn of the brushes in a given direction. Thus if the brush I2I had originally been in electrical contact with brush He at the instant the brushes I20 and I2I commenced rotating counterclockwise relative to the brush 41c, as viewed in Figs. 12 and 13, relay SR2c would have been initially energized but in the interim following the breaking of electrical contact between brushes He and I2I and the contacting of brush I20 with brush "0, the relay SRZc is released so that whenever there is such counterclockwise relative rotation of the brushes I20 and I 2|, such prior energization of the relay SR2c is without effect and the sequence ofoperations of the control unit CUc does not become effective until the relays SRIc and SR3 have been first energized followed by the energization of the relay SE20.

When there is clockwise rotation of the brushes I20 and I2I relative to the brush "0, Figs. 12 and 13, however, the operative sequence does not commence until the relay SE20 has first been energized upon contact of the brush I2I with the brush 41c for the reason that under these conditions relays SRIc and SR3 will always be deenergized at the time relay SR2c is initially energized. If brush I20 then moves into contact with the brush 410 within the release time of the relay SRZc, relay SR3 will not be energized inasmuch as relay SR2c maintains its contacts 640 and 65c separated so that no energizing circuit can be established through the conductor 630 to the winding I24 of relay SR3, and unless relay SR2c releases while brushes I20 and "c are still in electrical contact, relay SR3 will not become energized during such clockwise relative rotation of the brushes I20 and HI. Relay SRIc energizes and closes its contact IIc when there is contact between the brushes 41c and I20 and if this occurs before the relay SE20 has been released, circuit is extended from the source Bo through the contact IIe of relay SRIc, contact 190 of relay SR2c, contacts 160 and I10 of relay SR3 to the conductor 36c which leads to a reverse field coil I2I of a motor as M, and this motor is thereupon actuated to correct the speed of its associated engine as E2, Fig. 1.

It may happen that the brushes I20 and I2I will successively traverse the brush 410 at a sufficient rate in one direction of relative rotation to set the control unit CUc for correcting relative rotation in that direction and that operating conditions may undergo a subsequent change which produces an opposite relative rotation of the brushes, in which event the setting of the control unit CUc will eiTect an operative lag until such setting is subsequently changed upon the completion of not greater than approximately one full turn of relative rotation in the new direction. Even under extreme conditions, however, such operative lag will be of only slight duration so that it will not interfere with the successful operation of the apparatus as has been explained hereinabove in connection with the foregoing embodiments of the invention.

The arrangement in Figs. 15, 16 and 17 is identical with that shown in Figs. 9, 10 and 11 in all respects, except that slow-to-release relays SR2li and SR3d have been substituted for the quickacting relays RI and R2. Also, the conductor 23d is in this instance connected to a terminal of the contact 6d of the relay SRZd and the conductor lid is connected to a terminal of the contact I0ld of the relay SR3d. When the direction of rotation of the brushes 85d, 86d, and 01d is counterclockwise relative to the master brush "d as viewed in Figs. 15 and 16, the brush 41d may originally be in electrical contact with brush 81d but while brush it is passing from the brush 01d to the brush 85d, the relay SR3d if it was originally I Electrical contact of the brushes 41d and 85d establishes a circuit from the source of voltage Ed through these brushes and conductor 62a. to one terminal of the normally closed contact 88d of the relay SR3d and inasmuch as this relay is deenerglzed circuit continues through the contact 88d and conductor 88d, terminal 90d and conductor 9Id to the winding I25 of the relay SR2d and thence through conductor 93d to the voltage source Bd. Relay SRZd thereupon energizes to open its contact 94d and closes contacts 95d and lid. Continued relative rotation of the brushes as aforesaid brings the brushes 41d and 86d into electrical contact to thereby extend circuit from the source Bd to the conductor 6Id for energizing the winding of the relay SRId. If this occurs before relay SRZd has released, a holding circuit for relay SRZd is established through the contact I00d of relay SRId and contact 95d of relay SR2d to the winding I25 of the latter relay, and hence the relay SR2d remains locked up so long as the relay SRId does not release. As relative rotation continues, brush 81d comes into electrical contact with brush 41d and circuit is extended from the voltage source Bd through the brushes to the conductor 63d, which leads to one terminal of the contact 9411 of the relay SR2d. However, if thisoccurs while the relay SR2d still remains locked up as aforesaid, circuit cannot be completed beyond the contact 94d inasmuch as this contact is held open when the relay SR2d is energized. Thus, in effect, relay SR3d is prevented from energizing so long as relay SRId remains energized during counterclockwise relative rotation of the brushes of the switch 52d, Fig. 16.

Energization of the relays SRId and SRM completes an energizing circuit from the source of voltage Bd through the contact Nd and lid of these relays to the conductor 23d for energizing a forward field coil as 20 of a motor as M, Fig. 2, and so long as counterclockwise rotation of brushes 85d, 86d and 81d relative to the brush 41d continues as viewed in Figs. 15 and 16, the slow-to-re1ease relays SRld and SRM are alternately energized, to assume a concurrently operative condition either continuously or intermittently, depending upon the degree of relative velocity of the brushes, to either continuously or intermittently energize the motor field coil as 20 until a state of substantial synchronization is reached.

When there is clockwise rotation of the brushes 05d, 86d and 81d relative to the brush "d, the

relays SRld, SRZd and SR3d will initially be in a deenergized state shortly prior to the establishment or electrical contact between the brushes "ii and lld, but when such contact is established circuit is extended from the voltage source Bd through these brushes to the conductor lld and thence through the normally closed contact lld of relay SRld, conductor ll id, terminal illd, and conductor llld, to the winding Ill of relay SR and thence through conductor llld, terminal lld and conductor Ild back to the source 01' voltage Bd so that relay SR is ener to maintain its contact lld open and its contact llld and "Id closed for a predetermined time after the brush lld moves out of electrical contact with the brush lld. If the degree of relative rotation is great enough so that brush 4111 comes into electrical contact with the brush lld during this interval, circuit is established through these brushes for energizing the relay BRld and upon energization of this relay a holding circuit is established for the winding ill of relay BRld through the contact llld this relay and the contact illd of relay BRld so long as the latter relay remains in a state of energization. Opening of the contact lld upon energization oi the relay SRld prevents the relay SRId from energizing when brushes 41d and lld come into electrical contact, provided relay SRld is still energized to maintain the holding circuit for the relay BRld. Concurrent operation or the relays SRld and SR completes circuit from source of voltage Bd through the contacts lid and ll'ld of these relays to the conductor lld which leads to a reverse field coil as II o! a motor asM, Fig. 2, and this circuit is either maintained continuously or is repeatedly established for every relative turn of the brushes in switch Bid with respect to the master brush lld in the switch Sid, depending upon the value of the relative velocity, until the motor M has brought the engine which it controls back into substantial synchronism with the master engine.

It is desirable when the brushes oi the switch 82:! are rotating counterclockwise relative to the master brush lld, that the relay BRld be prevented i'rom energizing. 01' course, there is always the possibility that there may be a slight clockwise relative rotation of the brushes lld and lld which is sumcient to cause energization and locking up oi the relay SRld and that the relative rotation oi the brushes may then he suddenly reversed so that the brushes lld and "it commence to turn counterclockwise relative to the master brush 41d and under these circumstances the relay SRld may remain energized for a brief period during suchcounterclockwise rotation relative to the master brush lid. The release time of relay SR! is sumciently limited so that this relay releases and breaks the holding circuit to the winding of the relay SRld on the first counterclockwise relative turn or the brushes oi the switch 82d, even under extreme conditions, at least by the time brush lld has moved into contact with the brush lld so that the operative delay eiiected as a result or the energized condition oi the relay SRld is promptly rectified when relay SRld releases and closes its contact lld, to thereby enable circuit to be extended from the source of voltage Bd through the brushes 41d and lid to the winding of relay SR!!! and thereafter the operation of the control unit CUd is the same as described hereinabove for relative counzerclockwise rotation of the brushes lld, lld and d.

In all the foregoing embodiments oi my invention, the control unit such as CU, Fig. 5, has in cluded therein at least one relay such as SR2 which is common to the means for determining the direction of relative rotation 01' the brushes and the means responsive to the magnitude 0! such relative rotation. Thus in Fig. 5 not only does the relay 8R2 cooperate with the relay R to condition circuit to one or the other 01 the field coils ll or 2i 0! a motor as M, Fig. 2, depending upon the sense 01 the relative rotations between the brushes oi the master switch as SI, Fig. 4, and the switch as S2, but relay SR2 also cooperates with the relay SRI to complete the thus conditioned circuit whenever there is a predetermined rate of such relative rotation. This renders the sensitivity of the relative direction determining means comparable to that 0! the means responsive to the magnitude of relative speeds of rotation oi! the engines as El and E2, Fig. 1. It may be desirable in some instances that the relative direction determining means be considerably more sensitive than is the means responsive to magnitude of relative rotation and for this purpose a circuit such as is exemplified in Fig. 18 might be utilized. This arrangement is generally equivalent to that shown in Figs. 4 and 5 except that each master engine as El, Fig. 1, has associated therewith at least two master switches as Big and Biz, and two commutator switches as 8211 and S22 are provided for each engine as E2. The segments of the switches as Big and 821/ are interconnected by a cable llu, and those of the switches SI: and S22 by a cable Hz. The brushes llu and 501/ of the switch S2 cooperate with the master brush ll Oi the switch Sly for operating the slcw-to-release relays BRlw and SE21 which are similar to and function for the same purpose as the relays SRI and SR2, Fig. 5, in so far as these relays are responsive to the magnitude of rotation of the brushes I81: and El]! relative to the brush 411/. The brushes 492 and 502 of the switch S22 cooperate with a master brush Hz to operate the relay: SR2: and R2 which are similar to and function the same as the relays SR! and R, Fig. 5, in so far as these relays are responsive to the sense of rotation of brushes Hz and 50.2 relative to the brush "2.

The brushes of the switches Siz and S22,- however, may be geared to rotate at a much higher speed than the brushes of the switches Sly and S211 and thus any discrepancy in engine speeds is, in effect, magnified in so far as the switches SI: and S22: are concerned. The release time of the relay SRZz can be so adjusted as to accommodate this increased sensitivity and afford a more rapid response to the relay Re to discrepancies in engine speeds. Hence the relay R2 will condition an energizing circuit to a selected one of the field coils or ii of the motor M, Fig. 2, sumciently far in advance of a time when the relays SRlu and SRZy are operated under control oi the switch S2 when there is a difference in relative rotative speeds of the rota-tive parts such as propellers PI and P2, Fig. 1, that all likelihood of operative delay is eliminated.

It may be desirable, particularly where there are a large number of engines or the like to be correlated, to reduce the number of conductors required in the cables such as 5i, Fig. 1, that interconnect the master switch as Sl with each oi the subordinate switches as S2. To this end. it will be feasible in some instances to arrange the conductors as shown in Fig. 19 so that each segment or each switch as SI/ or 82! isin electrical contact with the segment 180 degrees removed from it. This reduces the number conductors required in a cable as to one-half the number required in circuits as shown in Fig.

4. However, this also means that the master brush as 41; of the switch SI] is brought into electrical contact not only with those segments of a switch as S2,f with which it would be in electrical contact in an arrangement as shown in Fig. 4, but also with those segments of the switch as 82] which are 180 degrees removed from the first-named segments of this switch. This will necessitate a. somewhat different spacing of the brushes of a switch as S2! than would otherwise be required, and each relay as SRIc, Fig. 14, under control of one of these brushes will be energized a plurality of times (in the present instance, twice) in the course of a single turn of the master brush 41 relative to the brushes in the switch S2f. Taking for example the arrangement of the apparatus shown in Figs. 13 and 14, the brushes I20 and I 2| would with the new arrangement be placed closer together, as are the brushes I201 and HI and the master brush 411 can be likewise made narrower than the brush 470, Fig. 12, so that the relative arc transversed by the master brush "I from the instant it moves out of electrical contact with either of the brushes I20;f or I2If to the time when it moves into electrical contact with the other of these brushes (considering only the smaller arc) is appreciably less than 90 degrees. The relays as SRIc, SR2c and SR3, Fig. 14, which are under control of brushes as I20! and I2If, Fig. 19, have their release times adjusted to accommodate this small relative travel of the master brush 41 but will not respond to effect any premature energization of a relay as SR2c if, for example, the brush 41f being originally in electrical contact with the brush I20) then commences to rotate counterclockwise relative thereto, as viewed in Fig. 19, to thereby produce the same effect as though a diametrically opposite brush 41)" were to be brought into electrical contact with the brush I2If, the relative length of travel of a brush as 41]" from the position in which it is shown in Fig. 19, to the brush I2If being sufiiciently great under these circumstances so that the relays as SRIc and SR3 may deenergize on the first relative turn of the brushes before the brush 41f makes contact with the brush I2If.

As has been explained hereinabove in connection with the various embodiments of my invention, the release times of slow-to-release relays such as SR2, Fig. 5, or SRIb, Fig. 11, are so selected that each such relay is unable to remain in an operative condition during at least the first two successive electrical contacts established between the master brush and the brush to which such slow-to-release relay is electrically connected, upon initiation of contemplated relative rotations of the brushes. The time factor thus incorporated in the operation of these relays insures against misoperation of the control apparatus when there is a sudden reversal in the sense of relative rotation of the propellers under conditions which will be encountered in the normal use of aircraft. For example, in the embodiment illustrated in Figs. 9 to 11, the brushes 86 and 81 might be successively brought into electrical contact with the master brush 41 while rotating counterclockwise relative thereto, to thereby energize the relays SRIb and R2 for correcting a low rotative speed of the propeller as P2, Fig. 1, relative to the master propeller as PI, and there might then occur some change in operating conditions which causes the propeller P2 to rotate more rapidly than the propeller PI, so that the sense of relative rotation of the brushes is reversed. Hence, assuming relays SRIb and R2 to have been energized as aforesaid, the brushes 85, 88 and 01 thereupon commence to rotate clockwise relative to the master brush "b, as viewed in Figs. 9 and 10, and upon the establishment of electrical contact between brushes b and 86, relay SRIb is re-energized. If this occurs prior to the release of the relay SRIb following its previous energization, the relay R2 remains locked up through its holding circuit controlled by the contact I00 of relay SRIb, and relay RI is prevented from energizing, until such time as relay SRI b releases, notwithstanding that relay RI and not relay R2 should be energized under these circumstances.

It is contemplated that even under the most extreme conditions which will be encountered in practice, relay SRIb will be released prior to the instant when brush 86 electrically contacts brush 4'") a second time while rotating clockwise relative thereto, so that relay R2 will be released due to opening of the contact I00 of relay SRIb during the first relative clockwise turn of the brush 86. This enables relay Rl to subsequently become energized and to lock up, when relay SRIb is again energized and closes its contact I00, and relay R2 is thereafter prevented from energizing so long as relay SRIb is in an operative condition. However, it may be desirable to provide an absolute safeguard against the possibility, remote as it may be, that the relays SRIb and R2 will be caused to assume an operative condition for correcting low relative velocity of the propeller P2, and that the relative velocity of the propeller P2 may then increase at such a rate that brush 86 repeatedly contacts brush 41b, with suilicient rapidity to maintain the relay SRIb in a continuously operative condition, thereby holding the relay R2 in its locked-up condition and preventing the relay RI from energizing. Because of this, I have provided a form of my invention, as illustrated in Fig. 20, which positively insures against the occurrence of an improper condition as just described.

The control unit Cug shown in Fig. 20, is similar in many respects .to the control unit CUb, Fig. 11, and is adapted to be operated under the control of a commutator switch as 82b, Fig. 10. In the present instance, however, the conductor 51 leading from the winding 59g of the relay SRIg is not connected directly to the source of voltage Bg but leads to one terminal of the contact I30 of the relay RIg. The contact I30 is normally closed When relay RIg is deenergized and serves to connect the conductor 519 with a conductor I3I that leads to one terminal of a like contact I32 of the relay R2g, which contact normally connects the conductor I3I to a conductor I33 leading to a terminal I34, and thence circuit is completed through the conductor 109 to the terminal 509 of the voltage source By. Whenever a relay as Rig or R29 is energized, however, .the energizing circuit for the winding 59g of the relay SRIg is broken due to the opening of either of the contacts I30 or I32. Hence, if as in the above described example of operation, relay RZg is energized while the relay SRIg is in an operative condition, such as would occur when there is counterclockwise relative rotation of the brushes of the switch as S20, Fig. 10, the contact I" remains open so long as relay R2g is held locked up through its holding circuit, which is to say, so long as relay SRIg remains in an operative condition. Then, if the sense of relative rotation of the brushes should be suddenly reversed, and as brush 41b, Fig. 9, moves into electrical contact with the brush as 66, the relay SRig will nevertheless continue to deenergiae and will subsequently be released to thereby break the holding circuit to the relay Rlg regardless of how long or how often the brush as 86 is in electrical contact with the master brush as 41b when there is relative clockwise rotation of the brushes as 66. When relay R20 has been released to close its contact I32, the energizing circuit for the relay SRIg is again conditioned for operation and thereafter the relay SRIg and then the relay Rig may become energized to bring about .the necessary control operation for correcting the high relative speed of the propeller as P2, Fig. 1.

To aflord still further reliability of operation, the conductors g and I060 leading from the windings 02g and i04g, respectively, of the relays Rig and R2g have been connected to the condoctor .60 which leads to one terminal of the normally open contact I00g of the relay SRIg, the other terminal of this contact being connected by a conductor I36 to the terminal 66g of the voltage source Bg. The conductors 91g and I000 leading from corresponding terminals of the normally open holding-circuit contacts 06g and I06g of the relays Rig and Rig, respectively, are connected to a conductor I36 leading to the terminal 55g of the voltage source By. This arrangement insures that neither the relay Rig nor the relay Rig can be energized so long as the relay SRI g is in an inoperative condition, and hence, all control operations must begin with the operation of relay SRIg.

For example, if upon initiation of counterclockwise relative rotation of the brushes of the switch as 82b, Fig. 10, the master brush as 41b should first be brought into electrical contact with the brush as 06, the relay Rig will not be energized inasmuch as the energizing circuit through its winding 92g cannot be completed until the contact I00g of relay SRIg is closed, which is to say, until relay SRIg is energized to assume an operative condition. If, however, the relay SRIg becomes energized upon subsequent contacting of the brush as 66 with the master brush as 41b, and while SRIg is maintaining its contact l00g closed the brush as 01 should electrically contact the master brush as 41b, circuit from .the voltage source B9 is extended through the brushes as 41?) and 61, conductor 63g, closed contact 94g of the deenergized relay Rig, and conductors Mia and I030 to the winding I04g of relay R2g, and thence through conductors Ig and 999, closed contact I000 of the relay SRIg, and conductor I back to the voltage source Bg. Relay R20 thereupon energizes and closes its contact N60 to thereby establish a holding circuit from the terminal 66g of the source By through the conductors I 36 and I 06, contact I 069, and condutcor I009 to the winding I04g, which holding circuit continues through the contact 400g of relay SRlg as just described, so long as relay SRIg maintains its contact closed. Energization of the relay R2g opens the contact 60g to thereby prevent the relay Rig from energizing. as in the embodiment shown in Fig. 11, and also opens the contact I32 which is eiiective as aforesaid to prevent the relay SR'Ig from being subsequently energized until it has first assumed an intervening inoperative condition.

It will be appreciated from the foregoing that relay SRIg cannot remain operative formore than a predetermined time following each energizetion thereof, unless it. should so happen that a condition of substantial synchronism is attained while the brushes as 41b and 66 are in electrical contact. Therefore, irrespective of how great the magnitude of the relative velocity of the propellers as PI and P2 may be, the relay SRIg cannot remain in a continuously operative condition and hence even at high relative velocities, the control unit CUg may furnish a series of control impulses to the motor as M, Fig. 2, although the amount of overlap in the release time of the relay SRIg following the subsequent energization and locking up of a relay as Rig or Rlg will vary in accordance with the magnitude of such relative rotation as in the case of the control unit cm, Fig. 11.

It may be desirable to eliminate the pulsing control afl'orded by the control unit CUg, Fig. 20, at high relative speeds of the propellers without affecting the pulsing control at lower speed differentials, which, as has been explained hereinaibove, aids to prevent hunting when the propellers are being brought into substantial synchronlsm. To this end, I have shown in Fig. 21 a modification of the circuit arrangement disclosed in Fig. 20. This modification consists of incorporating the slow-to-release relay SR4 in the circuit for maintaining the connection from the terminal 5671. of the voltage source Bh to the winding 5972. of the slow-to-release relay SRIh continuously closed at high relative speeds. It will be understood that the control unit CUh, which is fragmentarily shown in Fig. 21, is identical in all respects with the control unit CUQ, except for the addition of the relay SR4, like parts bearing similar reference numbers differing only in the suffix added thereto.

The control unit CUh, isunder the control of a commutator switch S211 that is similar in construction to the switch 5212, Fig. 10, except that a fourth brush M0 is added to the rotor assembly, this brush being preferably located diametrically opposite the brush 06h which controls the relay SRIh. When the brush I40 is brought into electrical contact with the master brush as 41b, Fig. 9, circuit is completed from the voltage source Bh through said master brush and the brush I40, Fig. 21, to the conductor I that leads to one end of the winding I42 of the slow-to-release relay SR4, thence through conductor I 40, terminal I44, and conductor I45 to a terminal I46 which is included in the conductor I05h leading back to the voltage source 1371.. Relay SR4 thereupon energizes and closes its normally open contact I41 to thereby establish an electric connection from the terminal 4 and conductor I48 to the conductor I49 which leads to the terminal I50 that is included in the conductor 51h connected to the end of the winding 69h of relay SRIh opposite the end of this winding electrically connected to the brush 86h. Relay SR4 has a slowto-release time such that battery will always be available to the winding 59h of the relay SRIh when the master brush as 41b, Fig. 9, makes electrical contact with brush I40, Fig. 21, at a rate greater than a predetermined value. Thus when the relative speed of the propellers rises above a predetermined value, the relays as Rig and R2g, Fig. 20, are no longer effective when 

